ns4.zdns.google has no NS records.
NS records for this zone are defined at zdns.google.
NS records for this zone are defined at zdns.google.
Question and response
Question
dig ns4.zdns.google. NS
Answer
The response has no answer records.
Authority
zdns.google. 300 SOA ns1.zdns.google. cloud-dns-hostmaster.google.com. 1 21600 3600 259200 300
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0